1. Field of the Invention
The present invention relates to a high density optical disk, and more particularly, to an optical disk having an improved structure so that information of over about 15 gigabytes can be recorded on an information recording surface thereof.
2. Description of the Related Art
In general, an optical disk is widely adopted as an information recording medium of an optical pickup apparatus for recording/reproducing information in a non-contact manner. The optical disk is divided into compact disks (CDs) and digital versatile disks (DVDs). The specifications of the CD and DVD are determined considering the compatibility and information recording density, which are shown below in Table 1.
Here, the diameter and the thickness of an optical disk indicate the outer appearance of the CD and DVD and are set to be of identical sizes considering the inner space of a compatible optical disk player, and the size of an optical disk tray for loading/unloading the optical disk.
To achieve an information recording capacity of about 4.7 GB in a DVD, the DVD is standardized to have a different wavelength used, a different track pitch (the distance between a track and an adjacent track) and a different minimum pit size from those of the CD. That is, by using a smaller wavelength light emitted from a light source of an optical pickup apparatus, for example, a semiconductor laser, of 635/650 nm, the size of an optical spot formed on an optical disk can be decreased so that the track pitch and the minimum pit size can be reduced.
Also, as it was difficult to manufacture an optical disk having a thickness of 1.2 mm when the CD was first developed, the thickness of the optical disk is set to 1.2 mm so that deterioration in reproduced signals due to foreign material adhering to the surface of a substrate is minimized.
If the thickness of the DVD is standardized to be the same as that of the CD, as the allowable error in inclination of the optical disk according to high density decreases, there is a high probability that an error will occur during recording/reproducing of information. In consideration of the above, the thickness of the DVD substrate is set to 0.6 mm. Here, as the DVD records signals digitally, deterioration of signals due to contamination of a surface of the optical disk can be processed by means of a circuit.
The DVD having the above specification can be divided into a single-sided single layer structure, a double-sided single layer structure, a single-sided dual layer structure and a double-sided dual layer structure, as shown FIGS. 1 through 4.
Referring to FIG. 1, a single-sided single layer DVD includes an information substrate 1 of a thickness of 0.6 mm having a recording surface la at which information signals are recorded on the surface opposite the surface where light L is input, a reflection layer 3 formed by being coated on the recording surface 1a, a protective layer 5 for preventing oxidation of or damage to the reflection layer 3, a combining layer 7 and a protective substrate 9 attached to the protective layer 5 through the combining layer 7 for making the entire thickness of the optical disk correspond to the specification and preventing warping of the information substrate 1. The optical disk of a single-sided single layer structure has an information recording capacity of 4.7 gigabytes.
Referring to FIG. 2, a double-sided single layer DVD includes first and second information substrates 11 and 21 having thicknesses of 0.6 mm, first and second reflection layers 13 and 23 formed by being coated on recording surfaces 11a and 21a of the first and second information substrates 11 and 21, respectively, and a combining layer 15 provided between the first and second reflection layers 13 and 23. The optical disk of a double-sided single layer structure has an information recording capacity of about 9.4 gigabytes, twice the information recording capacity of the optical disk of a single-sided single layer structure.
Referring to FIG. 3, a single-sided dual layer DVD includes a first information substrate 31, a translucent layer 33 formed by being coated on a first recording surface 31a of the first information substrate 31, a second information substrate 35 formed on the translucent layer 33 to have a thickness of about 40 xcexcm through a 2P (photo polymer) process and having a second recording layer 35a, a reflection layer 37 formed by being coated on the second recording surface 35a, a combining layer 38, and a protective substrate 39 having a thickness of about 0.6 mm combined by the combining layer 38 for protecting the first and second information substrates 31 and 35. In this case, the total information recording capacity recorded on the first and second recording surfaces 31a and 35a is about 8.5 gigabytes.
Referring to FIG. 4, a double-sided dual layer DVD includes two optical disks 41 and 45 where the protective substrate is excluded from the DVD of a single-sided dual layer structure of FIG. 3, and a combining layer 43 for combining these two optical disks 41 and 45. The double-sided dual layer DVD has an information recording capacity of 17 gigabytes, twice the capacity of the DVD of a single-sided dual layer structure.
The DVDs having one of the above structures maintains an overall thickness of 1.2 mm by combining 0.6 mm information substrate and/or protective substrate so that the information capacity is greater than that of a conventional CD.
When the thickness of the information substrate is standardized to 0.6 mm, by adopting a short wavelength optical pickup within a blue wavelength range (at 410 nm), high density information recording/reproducing at a level of 15 gigabyte per side is made possible. In this case, however, the allowable error in inclination of the optical disk which does not affect recording/reproducing sharply decreases. This is because the allowable amount of coma aberration according to the inclination of the optical disk is proportional to the wavelength, inversely proportional to the cube of the numerical aperture of an objective lens, and inversely proportional to the thickness of the optical disk.
Thus, even when the light source and the optical pickup apparatus having an objective lens satisfying the conditions needed for the ultra-high density information recording are adopted, when the DVD of the above structure is adopted as a recording medium, the allowable error in inclination of the optical disk decreases and thus the ultra-high density information recording/reproducing at a level of 15 gigabytes per side is not possible.
To solve the above problems, it is an objective of the present invention to provide an improved optical disk having the same size as that of a DVD or a CD with information capacity of 15 gigabytes or more per side.
Accordingly, to achieve the above objective, there is provided an optical disk comprising at least one information substrate having an incident surface where light for recording/reproducing signals is input and a recording surface where information signals are recorded, and at least one reflection layer formed by being coated on the recording surface for reflecting at least a part of incident light. In the above optical disk, assuming that the entire thickness of the optical disk is D and the distance between the incident surface and the recording surface of the information substrate is D1, the inequality 0.20xe2x89xa6D1/Dxe2x89xa60.38 is satisfied.
Also, to achieve the above object, there is provided an optical disk comprising a first information substrate having an incident surface where light for recording/reproducing signals is input and a first recording surface where information signals are recorded, a first reflection layer formed by being coated or the first recording surface for reflecting at least a part of incident light, and a second reflection layer formed on the second recording surface for reflecting at least a part of incident light. In the above optical disk, assuming that the entire thickness of optical disk is D and the distance between the incident surface and the first recording surface of the first information substrate is D1, the inequality 0.20xe2x89xa6D1/Dxe2x89xa60.38 is satisfied.
Also, to achieve the above object, there is provided an optical disk comprising a first information substrate having a first incident surface where light for recording/reproducing signals is input and a first recording surface where information signals are recorded, a first reflection layer formed by being coated on the first recording surface for reflecting a part of incident light, a second information substrate having a second incident surface where light for recording/reproducing signals is input and a second recording surface where information signals are recorded, a second reflection layer formed on the second recording surface for reflecting a part of incident light, a third information substrate formed by being combined between the first information substrate and the second information substrate and having a third recording surface where information signals are recorded/reproduced by light transmitted through the first information substrate and a fourth recording surface where information signals are recorded/reproduced by light transmitted through the second information substrate, a third reflection layer formed on the third recording surface for at least a part of incident light, and a fourth reflection layer formed on the fourth recording surface for at least a part of incident light. In the above optical disk, assuming that the entire thickness of the optical disk is D and each of the distances between the first incident surface and the first recording surface of the first information substrate and between the second incident surface and the second recording surface of the second information substrate is D1, the inequality 0.20xe2x89xa6D1/Dxe2x89xa60.38 is satisfied.